Different types of probes are known. These emit an incident beam of ultrasound towards the material to be analyzed. In order to obtain correct interpretations with good lateral resolution by analysis of the part of the beam that is reflected towards the probe from an internal interface it is necessary to have a convergent or parallel (i.e. collimated) incident beam which is as narrow and powerful as possible and reflection in the material without weakening by parasitic reflections and absorptions.
Instead, it is known that ultrasonic waves are reflected by any intermediate surface and very rapidly absorbed in gases, such as for example in air. These effects increase with the frequency of the waves. On the other hand, they propagate very well in liquid such as water.
In order to respond two to these two imperatives, all heretofore known probes thus comprise, between the emitter and the surface of the material to be analyzed, on the one hand, means generally in the form of lens as in optics, but in that case creating as many intermediate surfaces with parasitic reflection to ensure and monitor the convergence or collimation of the narrowest possible beam from the greatest source and, on the other hand, volumes which are full or filled or sprayed with water or gel ensuring continuity of propagation of the waves.
The two principal domains of application of this echographic equipment are the analysis of objects such as organs and skin in the medical domain, and plants in the agrifoodstuff domain, since, being for the major part composed of water, they lend themselves well to the penetration of the acoustic waves, which are, moreover, painless and a priori inoffensive, and the non-destructive inspection of materials in the mechanical domain, essentially for objects of metallic origin in which the acoustic waves also penetrate well.
Heretofore used probes have been the subject matter of a certain number of Patents: in particular, U.S. patent Ser. Nos. 400 547 and 551, filed on Jul. 21, 1982 and extended to Europe on Mar. 7, 1984 under Publication No. 0 102 179 by TECHNICARE CORPORATION, entitled "Selectable focus ultrasonic transducers for diagnostic imaging" and describing a concave emitter having a double curvature of focussing directly in a buffer transmission liquid. European Patent 0 33 751 is also known, published on Aug. 19, 1981 by HITACHI, entitled "Ultrasonic transducers using ultra high frequency" and claiming a length of the probe equal to a whole fraction 1/N of the Fresnel length. Finally, U.S. Pat. No. 3,934,460 may be mentioned, filed on Aug. 6, 1973 by GENERAL ELECTRIC COMPANY, entitled "Apparatus for focussing and collimating ultrasonic waves" and describing a probe comprising one or more lens, on one side filled with gel or liquid, and on the other side constituted by a material ensuring transmission up to contact with the object.
The majority of existing echographic probes use frequencies of 10 to 15 MHz maximum, which, on the one hand, avoids too great a reflection on the intermediate lens surfaces and parasitic absorptions in case of presence of absorbent space such as air, between the tip of the probe and the surface to be penetrated, as such absorption is a function of the square of the frequency, and, on the other hand, makes it possible to penetrate in the material up to a depth of 4 to 5 cm; a wave of frequency of 5 MHz penetrates for example up to 20 cm.
However, inversely, the resolution in depth is limited by the wave length, which, for low frequencies, does not make it possible to detect possible small defects, whilst the lateral resolution is linked with the width of the beam which, itself, is a function of the qualities of convergence of the probe.
Thus, although presently known equipment can analyze the materials at a depth of several centimeters, with sometimes even adjustments of focussing distance of the beam to improve the detection of echos at different depths, no equipment enables the frequency to be increased in order to effect analysis of the materials at very little depth, some millimeters at the most, beneath their surface with a very good resolution, both in depth and lateral.
The problem raised is that of effecting micro-echographic analyses essentially through deformable surfaces, avoiding the maximum of intermediate surfaces between the acoustic emitter and this surface and concentrating the beam at its end without aberration in order to obtain the most precise possible images of analysis.